1. Field of the Invention
The invention relates to a process for producing a component from a steel product coated with a protective Al—Si coating. The invention moreover relates to an intermediate product that arises during the course of such a process and that can be used to produce components of the type concerned here.
2. Description of the Related Art
Steel products of the type concerned here would typically be steel strips or sheets that are provided with an Al—Si coating in a known way, for example by hot-dip aluminising. The products concerned can, however, also be pre-formed, semi-finished products, which, for example, are pre-formed from sheet metal and then formed into the given finished product.
The Al—Si coating protects the component, formed from the given steel product, against corrosion during its period of use. The Al—Si coating nevertheless also provides an anti-corrosion effect, particularly protection against scaling, immediately following the coating of the steel substrate and maintains it during the deformation procedure. This particularly applies where the shaping occurs by means of what is known as “press hardening”.
In press hardening, the raw product to be shaped is brought, before shaping, to a temperature at which there is an at least partially austenitic structure and is then shaped while hot. The component obtained is then cooled in an accelerated manner either during the hot shaping procedure or immediately after it, in order to form a martensitic structure. Flat products, such as sheet-metal blanks or semi-finished products that have already been pre-formed or that are shaped at the end of the procedure, are used as raw product for the press hardening.
During the press hardening, the Al—Si coating prevents scales, which would considerably impede the shaping procedure, from forming on the steel product. In this way, it is possible to shape high-strength, heat-treatable steels that are exposed to particularly high levels of loading in the field.
A steel product typically used for this purpose is known in the field as “22MnB5”. Car body parts, which have to show a high level of strength even though they have a thin flat product thickness and are consequently comparably low in weight, are for example produced from steel products of this kind. Equally, other steel products, such as deep drawn steels of the type known under the trade name “DX55D” and composed in accordance with German industrial standard DIN EN 10327, and micro-alloy steels of the type alloyed in accordance with German industrial standard DIN EN 10292 and obtainable in the trade under the designation “HX300/340 LAD”, can nevertheless also be press mould hardened. It is also possible to use the raw products which according to the type of tailored blanks/patchwork blanks are made up of a plurality of sheets.
So that the Al—Si coating adheres so solidly for it not to break or peel during shaping, it is necessary for the steel product provided with the Al—Si coating to undergo heat treatment in which iron from the steel substrate is alloyed into the Al—Si coating. The aim here is to alloy the coating throughout its entire thickness to ensure that there are also no breaks or peeling off on the upper layers of the coating that abut against the free, outer side of the coated flat product. The type or level of full-layer alloying of Al—Si coatings moreover has an effect on the ease with which the components produced by press hardening can be welded and lacquered.
A process of the type described above is described in EP 1 380 666 A1. In this process, a steel sheet with an Al—Si coating is first heated to a temperature of 900° C. to 950° C., for 2 to 8 minutes. The coated steel sheet is then cooled to a temperature of 700-800° C. and is hot-shaped at this temperature. The shaped steel part is then quickly cooled to a temperature below 300° C. in order to produce a martensitic texture in the steel part obtained. The heat treatment of the steel substrate provided with the coating is carried out such that through diffusion of the iron from the steel substrate after the heat treatment the iron content in the coating lies between 80 and 95%. In this way, a hot-shaped component is to be obtained, combining good capacity for being welded, a good level of formability and a high level of corrosion protection.
One problem in carrying out the heat treatment that is necessary to obtain full-layer alloying is that, alongside setting a sufficient heating temperature, the product must also be left in the furnace for a certain time-period. The time-period for which the given steel product must be kept in the furnace is a function of the speed at which the substrate is heated, and of the necessary full-layer alloying of the substrate with the Al—Si layer. In the state of the art, the time in the furnace is from five to 14 minutes.
In practice, radiation furnaces are used for the heating, carried out before the hot-shaping, of the steel products provided with Al—Si coatings. Fundamental research on the behaviour under heating of steel products provided with Al—Si coatings in this context has shown that, in such furnaces, the reflection of the heat radiation from the surface of the given coating leads to a reduced heating speed by comparison with uncoated, or organically or inorganically coated, materials. Accordingly, a relatively long time-period has to be taken into account for the heating.
This long time-period leads to long processing times at the plant processing the flat products provided with an Al—Si coating, which increases not only the cycle times in producing the given component but also the equipment complexity of the furnace needed for the heating.
It would technically also be possible to heat the steel basis material of the flat products with its coating more quickly through inductive or conductive heating. The heating could also be accelerated by forced convection of the heat radiation. In the case of accelerated heating, there is nevertheless the risk that the alloying process in the Al—Si coating layer runs more slowly than the heating, with the result that the Al—Si layer is not fully alloyed or there are defects in the alloying. In an extreme case, the Al—Si layer may even run off the steel product.
An attempt is known, from DE 10 2004 007 071 B4, to reduce the processing time at the plant processing the flat products provided with an Al—Si coating by carrying out the full-layer alloying of the coating and the heating of the flat steel product to the relevant temperature in two separate stages. This approach enables the full-layer alloying process to be carried out with the manufacturer of the flat steel product provided with the Al—Si coating. The heating of the flat steel product provided with the coating which has already been full-layer alloyed can then take place at the plant, for example by means of induction or conduction, in an optimally short time-period and without needing to consider the formation of the coating. Accordingly, when using the known process, it is inherently possible to store flat steel products that have already been provided by the manufacturer with a full-layer alloyed coating in an intermediate storage facility, from which they can then be retrieved at short notice for further processing at the plant.
However, the proposal set out above is problematic in that the full-layer alloyed coating is itself subject to corrosion both during storage of the pre-produced flat steel products in the intermediate storage facility and also during the course of the working stages carried out at the plant. The problem arises from the iron content that is present on the exposed surface of the full-layer alloyed coating. In order to overcome such surface corrosion, costly protective measures are required that largely eat up the advantages gained in separating the full-layer alloying and press hardening. Added to this is the fact that cutting the flat product blanks coated with the full-layer alloyed coating, which cutting becomes necessary under certain circumstances before the hot-shaping, is difficult because full-layer alloyed Al—Si layers are hard and brittle. In view of the state of the art as outlined above, the object forming the basis of the invention was to create a process enabling shorter processing times at the plant for steel products provided with an Al—Si coating, without a risk of corrosion or disadvantages for subsequent cutting of the coated flat products having to be taken into account.